Isotactic polypropylene, as defined in Natta et al U.S. Pat. No. 3,112,300, is a polypropylene which consists essentially of isotactic macromolecules, i.e., macromolecules having substantially the isotactic structure and being insoluble in (non-extractable with) boiling n-heptane.
While said polypropylene is adapted to use in many commercially important applications, its impact strength at temperatures of 0.degree. C. or less is rather low, particularly for socalled commercial grade polypropylene.
Different ways of improving the impact strength of the polypropylene at low temperatures without unacceptable adverse effect on its other properties, including its flexural rigidity and thermal resistance have been proposed.
The technique which is most widely used for achieving that objective consists in polymerizing propylene in contact with a Ziegler/Natta stereospecific catalyst until most of the propylene is polymerized and then, during the final stage of the propylene polymerization feeding a different olefin, in particular ethylene, to the polymerization zone and continuing the polymerization until the amount of the added olefin, e.g., ethylene, polymerized is from 1% to 20% of the total (final) polymeric composition obtained.
U.S. Pat. No. 3,624,184 discloses a typical method which is widely followed. According to that method, propylene is first polymerized in an inert hydrocarbon solvent such as n-heptane and in the presence of a stereospecific polymerization catalyst prepared by mixing a Ti trihalide with a dialkyl Al monohalide to obtain a polymerization slurry, i.e., a suspension of polypropylene in the n-heptane. After "flash-off" of unreacted propylene until the slurry comprises a controlled amount of unreacted propylene, the slurry is preferably transferred to a second reactor, a mixture of ethylene and propylene in a molar ratio ranging, in general, from 1 to 6 is introduced into the second reactor, and the polymerization is continued until the amount of polymerized ethylene reaches a prefixed value which, generally, is from 5% to 20% by weight.
The main disadvantage of that process -- which in practice prevents conducting the polymerization continuously and involves many difficulties in batch polymerization -- is that during the polymerization of ethylene in the presence of propylene dissolved in the reaction medium, or in the presence of propylene fed in with the ethylene, considerable amounts of rubbery ethylene/propylene copolymers soluble in the reaction medium are formed, which give rise to considerable difficulties in the heat exchange and in transfer of the polymerization slurry.